This invention relates to streak camera devices, and more particularly to a streak camera device of photon-counting type.
Conventional techniques of observing a weak ultra-high-speed optical phenomena have been disclosed, for instance, by Japanese Patent Application Unexamined Publication No. 58745/1984, U.S. Pat. No. 4,797,747 assigned to the present assignee, and the Journal of the Institute of Television Engineers of Japan, Vol. 36, No. 11, pp. 1010-1012 (1982).
A photon-counting type streak camera device according to the above conventional techniques is as shown in FIG. 6.
As shown in FIG. 6, a pulse light source 1 comprising a mode-locked dye laser generates a pulse laser beam LB, which is split by a half-mirror HM into an exciting pulse beam LB.sub.1 and a trigger pulse beam LB.sub.2. The exciting pulse beam LB.sub.1 is reflected by mirrors in an optical delay section, so that it is applied to a specimen 3. As a result, the specimen 3 produces fluorescence light FL.
The fluorescence light FL passed through a slit S.sub.1 is focused on the photocathode 41 of a streak tube 4 by a lens L.sub.1, whereupon the photocathode 41 emits photoelectrons EB. The photoelectrons EB thus emitted pass through a grid 42, are accelerated by an accelerating electrode 43, and deflected by deflecting electrodes 44, so that they are applied to a microchannel plate (MCP) 45, where they are subjected to electron multiplication. The multiplied electrons are applied to a phosphor screen 46, thus forming a streak image 47.
On the other hand, the trigger pulse beam LB.sub.2 from the half-mirror HM is detected by a photodetector 5, and is applied through a variable delay circuit 6 to a trigger circuit 7, whereby a trigger signal is produced and applied to a sweep circuit 8. The timing of applying the exciting pulse beam LB.sub.1 to the specimen 3 and the timing of the trigger signal being outputted by the trigger circuit 7 are held in a certain relation by the optical delay section 2 and the variable delay circuit 6. Therefore, the position of the streak image 47 formed on the phosphor screen 46 corresponds to the period of time which elapses from the time instant when the exciting pulse beam LB.sub.1 is applied to the specimen 3 until the fluorescence light FL is generated. The streak image of the fluorescence light FL is formed on the phosphor screen 46 by illuminating the specimen 3 by the exciting pulse beam LB.sub.1, focusing the generated fluorescent light FL on the photocathode 41, and sweeping the photoelectrons EB. This operation is performed repeatedly, so that positions of the streak images 47 on the phosphor screen 46 are integrated. As a result, the weak high-speed optical phenomenon can be measured.
When, in the above-described system, the mode-locked dye laser forming the pulse light source 1 changes in power, then the streak image 47 on the phosphor screen 46 vibrates every sweep; that is, a so-called "jitter phenomenon" occurs. This is due to the following fact: When the laser power changes as indicated by P.sub.a and P.sub.b in FIG. 7(a), the output of the photodetector 5 is changed as indicated by V.sub.a and V.sub.b in FIG. 7(b), whereby the timing of the triggering of the streak camera is shifted as indicated by TRG.sub.a and TRG.sub.b, as a result of which the timing of sweeping with respect to the light pulse is changed as indicated by V.sub.sa and V.sub.sb in FIG. 7(c).
The above-described change of the timing, depending on the quality and adjustment of the laser, causes a jitter of several picoseconds to several tens of picoseconds. On the other hand, even if an ideal trigger signal is obtained with the power of the laser maintained unchanged, the electronic circuit in the streak camera has a jitter of several picoseconds to several tens of picoseconds, and suffers from a so-called "drift", i.e., the change in sweep timing which lasts over a relatively long period of time.
The jitter or drift causes no troubles when an optical phenomenon is measured by a single streak sweep; however, it will limit the time resolution in measurement in the case where it is required to measure a weak optical phenomenon with high S/N ratio by measuring it repeatedly with a time resolution of from several picoseconds to subpicoseconds, and integrating the results of measurement. For instance, in the case of a streak camera having a time resolution of 2 psec, with a jitter of 10 psec the time resolution of the system will be of the order of 11 psec.